Pressure sensors are commonly employed in automotive vehicle applications to control and monitor various aspects of vehicle operation. The pressure sensors are typically required to provide an accurate analog voltage output representative of the pressure applied to a sensing element. In automotive applications, the pressure sensor is generally required to be accurate over a large temperature range of approximately −40° to +125° C. throughout the life of the vehicle.
A typical pressure sensor is shown in FIGS. 1 and 2 including two components, namely a pressure sensing element shown in FIG. 1 and a compensation circuit shown in FIG. 2. The pressure sensing element 10 shown and described herein is a piezo-resistance sensor having four resistors R1-R4 configured in a Wheatestone Bridge. Input terminals 16 and 18 are coupled to voltage supply Vs and ground, respectively. Resistors R1 and R3 decrease in magnitude proportional to the applied pressure, and resistors R2 and R4 increase in magnitude proportional to the applied pressure. An increase in sensed pressure causes an increase in the voltage Vo+ on terminal 14, and a decrease in the voltage Vo− on terminal 12, thus producing a differential output voltage Vo+ minus Vo− that is proportional to the pressure applied to the sensing element 10. Pressure sensing elements have alternately been configured to include a variable capacitance type element.
The compensation circuit 20 shown in FIG. 2 can be a separate integrated circuit (IC) or may be integrated with the pressure sensing element 10 or other circuitry. The compensation circuit 20 receives the differential voltage inputs Vo+ and Vo− at terminals 14 and 12, respectively, and applies a differential voltage to a voltage-to-current converter and multiplier 22. In addition to converting the differential voltage to a current signal, the multiplier compensates for gain at room temperature and temperature dependent gain. This is achieved by controlling current sources IA and IB via a programmed function, such as lookup table 24. The temperature compensated current signal is then applied to a negative terminal of an amplifier 28.
The compensation circuit 20 also has a current source IS applied to the negative terminal of the amplifier 28. The current source IS compensates for sensor offsets at room temperature and temperature dependent sensor offsets. This is achieved by controlling current source IS via a programmed function, such as lookup table 26.
The resultant current is converted to an output voltage VOUT across the amplifier 28 and feedback resistor RFB. The resistor RLD applies current to place the output at a desired direct current (DC) offset. The resultant output voltage VOUT at output terminal 30 is the desired compensated output signal ranging in value between ground and supply voltage VDD. The output voltage VOUT is proportional to the pressure applied as an input to the sensing element 10. Linear errors in gain, offset, and temperature dependency are thus compensated with the compensation circuit 20.
With many pressure sensors, the sensing element is packaged in a module that is easily susceptible to module package stress. Such module package stress generally causes a differential voltage from the pressure sensor to produce non-linear temperature effects. The resultant stresses on the module exhibited with conventional pressure sensing modules typically change over the life of the sensor package and cause the output voltage VOUT signal to drift over time. The aforementioned non-linear temperature effects and the long-term drift generally cannot be easily calibrated out of the sensor arrangement, and thus will generally cause errors in the sensor output. These resultant errors limit the accuracy of the pressure sensor and complicate the sensor module design.
It is therefore desirable to provide for a pressure sensor module that experiences reduced or minimal package stress. It is further desirable to provide for such a pressure sensor module that experiences reduced or minimal non-linear temperature effects and sensor signal drift.